The present invention relates to self-powered radiation detectors, and more specifically to gamma flux responsive detectors. A self-powered radiation detector does not require a drive potential to generate a signal current. The signal current is the resultant difference between electron currents produced between the inner emitter electrode and the coaxial outer collector electrode as a result of the neutron or gamma flux interactions with the collector and emitter electrodes.
Gamma flux responsive self-powered detectors employ a low neutron cross-section, high atomic weight, high density emitter material such as platinum, lead, bismuth, tantalum, or tungsten. The collector material is also a low neutron cross-section material such as high nickel content steel. The gamma flux from a reactor produces an inward current from interaction with the collector electrode, and an outward current from the interaction of gamma photons with the emitter. The net difference between these currents is sensed as the signal current which is indicative of reactor condition.
The typical gamma responsive self-powered detectors of the prior art typically were fabricated with a solid platinum emitter, about 0.020 inch diameter being the typical emitter dimension.
It has been discovered that some improvement in response of a platinum emitter detector can be achieved by increasing the diameter of the emitter. The sensitivity of such platinum detectors can be doubled by increasing the emitter diameter to about 0.080 inch. Further analysis of such enlarged diameter emitters and of the electron signal producing mechanism have led the present inventors to a further improvement in detector sensitivity, as will be explained by reference to FIG. 3 which illustrates a conventional self-powered detector structure.
The electron current from the central emitter results from gamma rays travelling through the emitter and interacting at its far side to produce an outgoing electron. Gamma interaction on the incoming or incident side of the emitter produces electrons which travel further into the emitter and are unable to escape from the emitter. As the emitter diameter is increased, incident gamma rays have to travel a greater distance through the emitter material in order to reach the far side of the emitter. Since the emitter material has a high atomic weight and high density, the incident gamma rays will be attenuated in passing through the emitter material and there will be less outward electron current.